diff --git a/Cargo.lock b/Cargo.lock
index 5a986db..beea351 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -152,6 +152,7 @@ dependencies = [
  "anyhow",
  "cargo",
  "clap 4.0.29",
+ "libc",
  "prettyplease",
  "proc-macro2",
  "quote",
@@ -159,6 +160,7 @@ dependencies = [
  "serde",
  "serde_json",
  "syn",
+ "tempfile",
  "tracing",
  "tracing-subscriber",
  "tracing-tree",
@@ -736,9 +738,9 @@ checksum = "830d08ce1d1d941e6b30645f1a0eb5643013d835ce3779a5fc208261dbe10f55"
 
 [[package]]
 name = "libc"
-version = "0.2.134"
+version = "0.2.138"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "329c933548736bc49fd575ee68c89e8be4d260064184389a5b77517cddd99ffb"
+checksum = "db6d7e329c562c5dfab7a46a2afabc8b987ab9a4834c9d1ca04dc54c1546cef8"
 
 [[package]]
 name = "libgit2-sys"
diff --git a/Cargo.toml b/Cargo.toml
index 2b468c7..c8aaa20 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,7 +20,11 @@ rustfix = "0.6.1"
 serde = { version = "1.0.151", features = ["derive"] }
 serde_json = "1.0.90"
 syn = { version = "1.0.101", features = ["full", "visit", "visit-mut"] }
+tempfile = "3.3.0"
 tracing = "0.1.37"
 tracing-subscriber = { version = "0.3.16", features = ["env-filter"] }
 tracing-tree = "0.2.2"
 walkdir = "2.3.2"
+
+[target."cfg(unix)".dependencies]
+libc = "0.2.138"
diff --git a/src/build.rs b/src/build.rs
index a5d2d5f..6c40706 100644
--- a/src/build.rs
+++ b/src/build.rs
@@ -1,20 +1,42 @@
 use anyhow::{bail, Context, Result};
 use rustfix::diagnostics::Diagnostic;
 use serde::Deserialize;
-use std::{collections::HashSet, fmt::Display, path::PathBuf, process::Command, rc::Rc};
+use std::{
+    collections::HashSet,
+    fmt::{Debug, Display},
+    path::PathBuf,
+    process::Command,
+    rc::Rc,
+};
 
-use crate::{EnvVar, Options};
+use crate::{dylib_flag::RustFunction, EnvVar, Options};
 
 #[derive(Debug, Clone)]
 pub struct Build {
     inner: Rc<BuildInner>,
 }
 
+pub enum Verify {
+    Ice,
+    Custom(RustFunction),
+    None,
+}
+
+impl Debug for Verify {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Ice => write!(f, "Ice"),
+            Self::Custom(_) => f.debug_tuple("Custom").finish(),
+            Self::None => write!(f, "None"),
+        }
+    }
+}
+
 #[derive(Debug)]
 struct BuildInner {
     mode: BuildMode,
     input_path: PathBuf,
-    no_verify: bool,
+    verify: Verify,
     env: Vec<EnvVar>,
 }
 
@@ -40,18 +62,26 @@ impl Build {
             }
         };
 
+        let verify = if options.no_verify {
+            Verify::None
+        } else if let Some(func) = options.verify_fn {
+            Verify::Custom(func)
+        } else {
+            Verify::Ice
+        };
+
         Self {
             inner: Rc::new(BuildInner {
                 mode,
                 input_path: options.path.clone(),
-                no_verify: options.no_verify,
+                verify,
                 env: options.env.clone(),
             }),
         }
     }
 
     pub fn build(&self) -> Result<BuildResult> {
-        if self.inner.no_verify {
+        if let Verify::None = self.inner.verify {
             return Ok(BuildResult {
                 reproduces_issue: false,
                 no_verify: true,
@@ -59,7 +89,7 @@ impl Build {
             });
         }
 
-        let (reproduces_issue, output) = match &self.inner.mode {
+        let (is_ice, output) = match &self.inner.mode {
             BuildMode::Cargo { args } => {
                 let mut cmd = Command::new("cargo");
                 cmd.arg("build");
@@ -116,9 +146,15 @@ impl Build {
             }
         };
 
+        let reproduces_issue = match self.inner.verify {
+            Verify::None => unreachable!("handled ealier"),
+            Verify::Ice => is_ice,
+            Verify::Custom(func) => func.call(&output),
+        };
+
         Ok(BuildResult {
             reproduces_issue,
-            no_verify: self.inner.no_verify,
+            no_verify: false,
             output,
         })
     }
@@ -144,8 +180,6 @@ impl Build {
                     .into_iter::<CargoJsonCompileMessage>()
                     .collect::<Result<Vec<_>, _>>()?;
 
-                
-
                 messages
                     .into_iter()
                     .filter(|msg| msg.reason == "compiler-message")
diff --git a/src/dylib_flag.rs b/src/dylib_flag.rs
new file mode 100644
index 0000000..25c1786
--- /dev/null
+++ b/src/dylib_flag.rs
@@ -0,0 +1,116 @@
+//! Handles the --verify-fn flag.
+//! It takes in a Rust closure like `|str| true` that takes in a `&str` and returns a bool.
+
+use std::{fmt::Debug, str::FromStr};
+
+use anyhow::{Context, Result};
+use quote::quote;
+
+type Entrypoint = unsafe extern "C" fn(*const u8, usize) -> bool;
+
+#[derive(Clone, Copy)]
+pub struct RustFunction {
+    func: Entrypoint,
+}
+
+impl FromStr for RustFunction {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Self::compile(s)
+    }
+}
+
+fn wrap_func_body(func: &str) -> Result<String> {
+    let closure = syn::parse_str::<syn::ExprClosure>(func).context("invalid rust syntax")?;
+
+    let tokenstream = quote! {
+        #[no_mangle]
+        pub extern "C" fn cargo_minimize_ffi_function(ptr: *const u8, len: usize) -> bool {
+            match ::std::panic::catch_unwind(|| __cargo_minimize_inner(ptr, len)) {
+                Ok(bool) => bool,
+                Err(_) => ::std::process::abort(),
+            }
+        }
+
+        fn __cargo_minimize_inner(__ptr: *const u8, __len: usize) -> bool {
+            let __slice = unsafe { ::std::slice::from_raw_parts(__ptr, __len) };
+            let __str = ::std::str::from_utf8(__slice).unwrap();
+
+            (#closure)(__str)
+        }
+    };
+
+    Ok(tokenstream.to_string())
+}
+
+impl RustFunction {
+    #[cfg(not(unix))]
+    pub fn compile(body: &str) -> Result<Self> {
+        Err(anyhow::anyhow!("--verify-fn only works on unix"));
+    }
+
+    #[cfg(unix)]
+    pub fn compile(body: &str) -> Result<Self> {
+        use anyhow::bail;
+        use std::io;
+        use std::process::Command;
+        use std::{ffi::CString, os::unix::prelude::OsStringExt};
+
+        let file = tempfile::tempdir()?;
+
+        let full_file = wrap_func_body(body)?;
+
+        let source_path = file.path().join("source.rs");
+
+        std::fs::write(&source_path, &full_file).context("writing source")?;
+
+        let mut rustc = Command::new("rustc");
+        rustc.arg(source_path);
+        rustc.args(["--crate-type=cdylib", "--crate-name=helper", "--emit=link"]);
+        rustc.current_dir(file.path());
+
+        let output = rustc.output().context("running rustc")?;
+        if !output.status.success() {
+            let stderr = String::from_utf8(output.stderr)?;
+            bail!("Failed to compile code: {stderr}");
+        }
+
+        let dylib_path = file.path().join("libhelper.so");
+
+        let os_str = dylib_path.into_os_string();
+        let vec = os_str.into_vec();
+        let cstr = CString::new(vec)?;
+
+        let dylib = unsafe { libc::dlopen(cstr.as_ptr(), libc::RTLD_LAZY) };
+
+        if dylib.is_null() {
+            bail!("failed to open dylib: {}", io::Error::last_os_error());
+        }
+
+        let symbol = b"cargo_minimize_ffi_function\0";
+
+        let func = unsafe { libc::dlsym(dylib, symbol.as_ptr().cast()) };
+
+        if func.is_null() {
+            bail!("didn't find entrypoint symbol");
+        }
+
+        let func = unsafe { std::mem::transmute::<*mut _, Entrypoint>(func) };
+
+        Ok(Self { func })
+    }
+
+    pub fn call(&self, output: &str) -> bool {
+        let ptr = output.as_ptr();
+        let len = output.len();
+
+        unsafe { (self.func)(ptr, len) }
+    }
+}
+
+impl Debug for RustFunction {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("RustFunction").finish_non_exhaustive()
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index d3a599a..57f1218 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -4,6 +4,7 @@ extern crate tracing;
 use std::{path::PathBuf, str::FromStr};
 
 mod build;
+mod dylib_flag;
 mod everybody_loops;
 mod expand;
 mod privatize;
@@ -11,9 +12,10 @@ mod processor;
 
 use anyhow::{Context, Result};
 use clap::Parser;
+use dylib_flag::RustFunction;
 use processor::Minimizer;
 
-use crate::{processor::Processor};
+use crate::processor::Processor;
 
 #[derive(clap::Parser)]
 #[command(version, about, name = "cargo", bin_name = "cargo")]
@@ -33,6 +35,8 @@ pub struct Options {
     rustc: bool,
     #[arg(long)]
     no_verify: bool,
+    #[arg(long)]
+    verify_fn: Option<RustFunction>,
 
     #[arg(long)]
     env: Vec<EnvVar>,
diff --git a/src/processor/files.rs b/src/processor/files.rs
index e9ad284..3e1fae2 100644
--- a/src/processor/files.rs
+++ b/src/processor/files.rs
@@ -1,50 +1,40 @@
 use anyhow::{Context, Result};
-use std::{
-    fs,
-    path::{Path, PathBuf},
-};
-
+use std::{fs, path::{Path, PathBuf}};
 #[derive(Debug, PartialEq, Eq, Clone, Hash)]
-pub struct SourceFile {
-    pub path: PathBuf,
+pub(crate) struct SourceFile {
+    pub(crate) path: PathBuf,
 }
-
 #[derive(Default)]
-pub struct Changes {
+pub(crate) struct Changes {
     any_change: bool,
 }
-
-pub struct FileChange<'a, 'b> {
-    pub path: &'a Path,
+pub(crate) struct FileChange<'a, 'b> {
+    pub(crate) path: &'a Path,
     content: String,
     changes: &'b mut Changes,
     has_written_change: bool,
 }
-
 impl FileChange<'_, '_> {
-    pub fn before_content(&self) -> &str {
+    pub(crate) fn before_content(&self) -> &str {
         &self.content
     }
-
-    pub fn write(&mut self, new: &str) -> Result<()> {
+    pub(crate) fn write(&mut self, new: &str) -> Result<()> {
         self.has_written_change = true;
-        fs::write(self.path, new).with_context(|| format!("writing file {}", self.path.display()))
+        fs::write(self.path, new)
+            .with_context(|| format!("writing file {}", self.path.display()))
     }
-
-    pub fn rollback(mut self) -> Result<()> {
+    pub(crate) fn rollback(mut self) -> Result<()> {
         assert!(self.has_written_change);
         self.has_written_change = false;
         fs::write(self.path, &self.content)
             .with_context(|| format!("writing file {}", self.path.display()))
     }
-
-    pub fn commit(mut self) {
+    pub(crate) fn commit(mut self) {
         assert!(self.has_written_change);
         self.has_written_change = false;
         self.changes.any_change = true;
     }
 }
-
 impl Drop for FileChange<'_, '_> {
     fn drop(&mut self) {
         if self.has_written_change {
@@ -55,9 +45,8 @@ impl Drop for FileChange<'_, '_> {
         }
     }
 }
-
 impl SourceFile {
-    pub fn try_change<'file, 'change>(
+    pub(crate) fn try_change<'file, 'change>(
         &'file self,
         changes: &'change mut Changes,
     ) -> Result<FileChange<'file, 'change>> {
@@ -71,9 +60,8 @@ impl SourceFile {
         })
     }
 }
-
 impl Changes {
-    pub fn had_changes(&self) -> bool {
+    pub(crate) fn had_changes(&self) -> bool {
         self.any_change
     }
 }
diff --git a/src/processor/mod.rs b/src/processor/mod.rs
index 65fa6b1..b611bc0 100644
--- a/src/processor/mod.rs
+++ b/src/processor/mod.rs
@@ -1,19 +1,13 @@
 mod files;
 mod reaper;
-
 use std::{borrow::Borrow, collections::HashSet, ffi::OsStr, fmt::Debug, mem, path::Path};
-
 use anyhow::{Context, Result};
-
 use crate::{build::Build, processor::files::Changes};
-
-pub use self::files::SourceFile;
-
-pub trait Processor {
+pub(crate) use self::files::SourceFile;
+pub(crate) trait Processor {
     fn refresh_state(&mut self) -> Result<()> {
         Ok(())
     }
-
     /// Process a file. The state of the processor might get invalidated in the process as signaled with
     /// `ProcessState::FileInvalidated`. When a file is invalidated, the minimizer will call `Processor::refersh_state`
     /// before calling the this function on the same file again.
@@ -23,33 +17,27 @@ pub trait Processor {
         file: &SourceFile,
         checker: &mut PassController,
     ) -> ProcessState;
-
     fn name(&self) -> &'static str;
 }
-
 impl Debug for dyn Processor {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.write_str(self.name())
     }
 }
-
 #[derive(Debug, PartialEq, Eq)]
-pub enum ProcessState {
+pub(crate) enum ProcessState {
     NoChange,
     Changed,
     FileInvalidated,
 }
-
 #[derive(Debug)]
-pub struct Minimizer {
+pub(crate) struct Minimizer {
     files: Vec<SourceFile>,
     build: Build,
 }
-
 impl Minimizer {
-    pub fn new_glob_dir(path: &Path, build: Build) -> Self {
+    pub(crate) fn new_glob_dir(path: &Path, build: Build) -> Self {
         let walk = walkdir::WalkDir::new(path);
-
         let files = walk
             .into_iter()
             .filter_map(|entry| match entry {
@@ -67,54 +55,41 @@ impl Minimizer {
                 println!("- {}", file.path.display());
             })
             .collect();
-
         Self { files, build }
     }
-
-    pub fn run_passes<'a>(
+    pub(crate) fn run_passes<'a>(
         &self,
         passes: impl IntoIterator<Item = Box<dyn Processor + 'a>>,
     ) -> Result<()> {
         let inital_build = self.build.build()?;
         println!("Initial build: {inital_build}");
         inital_build.require_reproduction("Initial")?;
-
         for mut pass in passes {
             self.run_pass(&mut *pass)?;
         }
-
         Ok(())
     }
-
     fn run_pass(&self, pass: &mut dyn Processor) -> Result<()> {
         let mut invalidated_files = HashSet::new();
-
         let mut refresh_and_try_again = false;
-
         loop {
             let span = info_span!("Starting round of pass", name = pass.name());
             let _enter = span.enter();
-
             let mut changes = Changes::default();
-
             for file in &self.files {
                 if invalidated_files.contains(file) {
                     continue;
                 }
-
                 self.process_file(pass, file, &mut invalidated_files, &mut changes)?;
             }
-
             if !changes.had_changes() {
                 if !refresh_and_try_again && !invalidated_files.is_empty() {
-                    // A few files have been invalidated, let's refresh and try these again.
                     pass.refresh_state().context("refreshing state for pass")?;
                     invalidated_files.clear();
                     refresh_and_try_again = true;
                     println!("Refreshing files for {}", pass.name());
                     continue;
                 }
-
                 println!("Finished {}", pass.name());
                 return Ok(());
             } else {
@@ -122,7 +97,6 @@ impl Minimizer {
             }
         }
     }
-
     fn process_file<'file>(
         &self,
         pass: &mut dyn Processor,
@@ -131,29 +105,19 @@ impl Minimizer {
         changes: &mut Changes,
     ) -> Result<()> {
         let mut checker = PassController::new();
-
         loop {
-            dbg!(&checker);
-
+            dbg!(& checker);
             let file_display = file.path.display();
-
             let mut change = file.try_change(changes)?;
-
             let mut krate = syn::parse_file(change.before_content())
                 .with_context(|| format!("parsing file {file_display}"))?;
-
             let has_made_change = pass.process_file(&mut krate, file, &mut checker);
-
             match has_made_change {
                 ProcessState::Changed | ProcessState::FileInvalidated => {
                     let result = prettyplease::unparse(&krate);
-
                     change.write(&result)?;
-
                     let after = self.build.build()?;
-
                     println!("{file_display}: After {}: {after}", pass.name());
-
                     if after.reproduces_issue() {
                         change.commit();
                         checker.reproduces();
@@ -161,7 +125,6 @@ impl Minimizer {
                         change.rollback()?;
                         checker.does_not_reproduce();
                     }
-
                     if has_made_change == ProcessState::FileInvalidated {
                         invalidated_files.insert(file);
                     }
@@ -171,50 +134,35 @@ impl Minimizer {
                     checker.no_change();
                 }
             }
-
             if checker.is_finished() {
                 break;
             }
         }
-
         Ok(())
     }
 }
-
 #[derive(Clone, PartialEq, Eq, Hash)]
 struct AstPath(Vec<String>);
-
 impl Borrow<[String]> for AstPath {
     fn borrow(&self) -> &[String] {
         &self.0
     }
 }
-
 impl Debug for AstPath {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "AstPath({:?})", self.0)
     }
 }
-
 #[derive(Debug)]
-pub struct PassController {
+pub(crate) struct PassController {
     state: PassControllerState,
 }
-
 #[derive(Debug)]
 enum PassControllerState {
-    InitialCollection {
-        candidates: Vec<AstPath>,
-    },
-
-    Bisecting {
-        current: HashSet<AstPath>,
-        worklist: Vec<Vec<AstPath>>,
-    },
-
+    InitialCollection { candidates: Vec<AstPath> },
+    Bisecting { current: HashSet<AstPath>, worklist: Vec<Vec<AstPath>> },
     Success,
 }
-
 impl PassController {
     fn new() -> Self {
         Self {
@@ -223,44 +171,41 @@ impl PassController {
             },
         }
     }
-
     fn reproduces(&mut self) {
         match &mut self.state {
             PassControllerState::InitialCollection { .. } => {
-                self.state = PassControllerState::Success
+                self.state = PassControllerState::Success;
             }
-            PassControllerState::Bisecting {
-                current, worklist, ..
-            } => match worklist.pop() {
-                Some(next) => *current = next.into_iter().collect(),
-                None => {
-                    self.state = PassControllerState::Success;
+            PassControllerState::Bisecting { current, worklist, .. } => {
+                match worklist.pop() {
+                    Some(next) => *current = next.into_iter().collect(),
+                    None => {
+                        self.state = PassControllerState::Success;
+                    }
                 }
-            },
+            }
             PassControllerState::Success => unreachable!("Processed after success"),
         }
     }
-
     fn does_not_reproduce(&mut self) {
         match &mut self.state {
             PassControllerState::InitialCollection { candidates } => {
                 let candidates = mem::take(candidates);
                 let half = candidates.len() / 2;
                 let (first_half, second_half) = candidates.split_at(half);
-
-                self.state = PassControllerState::Bisecting {
+                self
+                    .state = PassControllerState::Bisecting {
                     current: first_half.iter().cloned().collect(),
                     worklist: vec![second_half.to_owned()],
                 };
             }
             PassControllerState::Bisecting { current, worklist } => {
-                dbg!(&current, &worklist);
+                dbg!(& current, & worklist);
                 todo!();
             }
             PassControllerState::Success => unreachable!("Processed after success"),
         }
     }
-
     fn no_change(&mut self) {
         match &self.state {
             PassControllerState::InitialCollection { candidates } => {
@@ -271,12 +216,13 @@ impl PassController {
                 self.state = PassControllerState::Success;
             }
             PassControllerState::Bisecting { current, .. } => {
-                unreachable!("No change while bisecting, current was empty somehow: {current:?}");
+                unreachable!(
+                    "No change while bisecting, current was empty somehow: {current:?}"
+                );
             }
             PassControllerState::Success => {}
         }
     }
-
     fn is_finished(&mut self) -> bool {
         match &mut self.state {
             PassControllerState::InitialCollection { .. } => false,
@@ -284,8 +230,7 @@ impl PassController {
             PassControllerState::Success => true,
         }
     }
-
-    pub fn can_process(&mut self, path: &[String]) -> bool {
+    pub(crate) fn can_process(&mut self, path: &[String]) -> bool {
         match &mut self.state {
             PassControllerState::InitialCollection { candidates } => {
                 candidates.push(AstPath(path.to_owned()));
@@ -298,43 +243,31 @@ impl PassController {
         }
     }
 }
-
 macro_rules! tracking {
     () => {
-        tracking!(visit_item_fn_mut);
-        tracking!(visit_impl_item_method_mut);
-        tracking!(visit_item_impl_mut);
-        tracking!(visit_item_mod_mut);
+        tracking!(visit_item_fn_mut); tracking!(visit_impl_item_method_mut);
+        tracking!(visit_item_impl_mut); tracking!(visit_item_mod_mut);
     };
     (visit_item_fn_mut) => {
-        fn visit_item_fn_mut(&mut self, func: &mut syn::ItemFn) {
-            self.current_path.push(func.sig.ident.to_string());
-            syn::visit_mut::visit_item_fn_mut(self, func);
-            self.current_path.pop();
-        }
+        fn visit_item_fn_mut(& mut self, func : & mut syn::ItemFn) { self.current_path
+        .push(func.sig.ident.to_string()); syn::visit_mut::visit_item_fn_mut(self, func);
+        self.current_path.pop(); }
     };
     (visit_impl_item_method_mut) => {
-        fn visit_impl_item_method_mut(&mut self, method: &mut syn::ImplItemMethod) {
-            self.current_path.push(method.sig.ident.to_string());
-            syn::visit_mut::visit_impl_item_method_mut(self, method);
-            self.current_path.pop();
-        }
+        fn visit_impl_item_method_mut(& mut self, method : & mut syn::ImplItemMethod) {
+        self.current_path.push(method.sig.ident.to_string());
+        syn::visit_mut::visit_impl_item_method_mut(self, method); self.current_path
+        .pop(); }
     };
     (visit_item_impl_mut) => {
-        fn visit_item_impl_mut(&mut self, item: &mut syn::ItemImpl) {
-            self.current_path
-                .push(item.self_ty.clone().into_token_stream().to_string());
-            syn::visit_mut::visit_item_impl_mut(self, item);
-            self.current_path.pop();
-        }
+        fn visit_item_impl_mut(& mut self, item : & mut syn::ItemImpl) { self
+        .current_path.push(item.self_ty.clone().into_token_stream().to_string());
+        syn::visit_mut::visit_item_impl_mut(self, item); self.current_path.pop(); }
     };
     (visit_item_mod_mut) => {
-        fn visit_item_mod_mut(&mut self, module: &mut syn::ItemMod) {
-            self.current_path.push(module.ident.to_string());
-            syn::visit_mut::visit_item_mod_mut(self, module);
-            self.current_path.pop();
-        }
+        fn visit_item_mod_mut(& mut self, module : & mut syn::ItemMod) { self
+        .current_path.push(module.ident.to_string());
+        syn::visit_mut::visit_item_mod_mut(self, module); self.current_path.pop(); }
     };
 }
-
 pub(crate) use tracking;